Aiming circle/guide in Anti Air Gun Emplacement

I was in an Anti Air Gun emplacement, when randomly a circle appeared ahead of a fast jet. This circle followed ahead of the fast jet, albeit glitchily, but when I used that as my aiming reference, it was highly accurate. 

It only appeared on two aircraft, not on all of them. One fast jet, one helicopter. 

When I died in the anti air emplacement, the circle remained around something fixed on the map, where I assume a fast jet had crashed. It didn't matter where I looked or turned, I could turn back and it would be there.